The present application relates to corrosion monitoring.
Corrosion causes enormous economic loss to industry. The ability to monitor the rate of corrosion allows corrective and preventative action to be taken and can minimise or alleviate this cost. One method of monitoring corrosion is to measure and analyse the naturally occurring currents and voltages that are generated in corroding materials.
Iverson (xe2x80x9cTransient Voltage changes produced in corroding metals and alloysxe2x80x9d, Electrochemical Science, June 1968) used a high impedance voltmeter and a filter to block DC voltage, to monitor this electrochemical voltage noise, and related this to the action of chemical inhibitor on a corroding electrode.
Eden, Hladky and John (xe2x80x9cCorrosion 86 Paper 274xe2x80x9d, March 1986) used a ZRA (zero resistance ammeter) to monitor electrochemical current noise flowing between two identical, corroding electrodes. This paper also discloses how, with the addition of a third voltage reference electrode (either of the same material, or of an inert, non-corroding material), current noise and voltage noise can be measured simultaneously. This paper also discloses various statistical methods to process the current and voltage noise, including moving the data from the time domain to the frequency domain using an FFT (fast Fourier transform).
Known electrochemical noise systems use a ZRA to measure current noise. This requires the two electrodes connected to the ZRA to be of similar material, so that they have the same galvanic potential. Connecting dissimilar materials to a ZRA will tend to produce artificially large circulating currents and corrosion, due to the difference in galvanic potential. Thus both electrodes are constructed of the same material as the pipe or vessel that is to be monitored. One disadvantage of this is that it is not possible to determine the direction of the current flow that is causing corrosion, as an anodic reaction on one electrode will produce the same reaction as a cathodic reaction on the other electrode. Furthermore, in practice, probes have a tendency to xe2x80x9cbridgexe2x80x9d with corrosion products and scale, and having to have at least two electrodes that are subject to corrosion encourages this failure mechanism.
The present invention seeks to enable electrochemical current noise to be continuously monitored using two dissimilar electrode materials, so that only one electrode need be constructed of the corrodable material. The present invention further seeks to enable electrochemical current and potential noise to be continuously monitored using at least 3 electrodes, only one of which is necessarily constructed of corrodable material.
According to the present invention, a corrosion monitor comprises electronic circuitry arranged such that DC current flowing between two electrodes is reduced to essentially zero, while allowing any naturally occurring AC current noise to flow unhindered and be monitored by the instrumentation.
The two electrodes consist of one inert reference electrode, and one electrode constructed of the material to be monitored (the working electrode). Even though the two electrodes will have different galvanic potentials, by reducing the DC current to zero the electronic circuitry is able to avoid galvanic effects.
Furthermore, the voltage potential can be monitored between the inert current reference electrode, and a third electrode also constructed of an inert material. As corrosion activity occurs on the working electrode, both current noise and voltage noise may then be monitored simultaneously.
It is important to note that only one electrode need be made of the material to be monitored, in which case all current and voltage noise is associated with the activity on this one electrode.
In practice, the corrosion monitor can comprise an inert reference electrode and a working electrode of the material to be monitored, and a voltage follower adapted to apply a voltage between the electrodes, which voltage reflects previous values of the current flowing between the electrodes. This voltage is preferably proportional to an integration of this current.
The current flowing between the electrodes can be measured and that output can then be fed to an integrating circuit This will produce an output which can be fed to the voltage follower for application to the working electrode. Measurement of the current can be by simply detecting the voltage drop across a resistance or other impedance.